The most common method of providing line isolation and electrical power grid backup is through the use of uninterruptible power supplies/sources (UPS) as a secondary power source for providing sufficient power until another secondary power source such as a motor generator is operational and providing stabilized power. This basic configuration is illustrated in FIG. 1. As shown, some electrical load 100 (e.g., a campus, office building, manufacturing facility, data center, residence, hospital, etc.) normally receives electrical power from a utility company via the electrical power grid 110. In the event that power cannot be provided by the utility, e.g., blackout conditions, demand response power reductions, etc., one or more generator based secondary power sources 120 are available. Here “DG” refers to available diesel generators. Precisely which power source is used at any particular time is controlled by power electronic switch 115. In the simplest example, power is switched from one or the other of utility grid 110 and secondary power source 120, but in other cases power can be delivered from some combination of the two. However, since secondary power sources such as generators typically take several seconds to several minutes to bring on line, and additional secondary power source (UPS 130) is used to provide short term but quickly available power until power from generators is available.
The most common types of motor generator units are internal combustion engines (ICs, whether gasoline, diesel or natural gas based), gas turbines (GTs), and microturbines (MTs). In addition to having finite start-up times, all of these types of motor generator units have various other drawbacks. For example, the generators are typically complex machines that require special operation and routine maintenance, their electrical efficiency is typically in the range of 20-40%, the equipment lifetime may be 10-20 years, they require combustible and sometimes dangerous fuels, they produce various emissions, and their availability (e.g., as affected by device failures) can be limited to 90-95%. Moreover, initial equipment costs, fuel costs, and maintenance costs can make such generators a very expensive secondary power source.
UPS systems use stored energy as a secondary power source to protect the critical load and provide sufficient time to switch motor generators on-line to assure no loss in power to the user. The reliability of this stored energy is fundamental to the reliability of the system. Due to their advantages in cost, energy storage density, discharge characteristics, and infrastructure, lead-acid batteries are the most commonly used type of stored energy in UPS systems today. In particular, valve-regulated lead-acid (VRLA) batteries are dominant in this application. However, despite battery manufacturers' best efforts to improve their products, experience has shown that the useful life of a VRLA battery array in conventional double-conversion UPS systems is two to three years. Beyond two years, cell failure rates quickly reach unacceptable levels.
Electrochemical batteries prematurely reach end of life for two reasons: manufacturing defects and battery management issues. Manufacturing defects include “cold” welds between adjacent cells; inter-cell shorts; reversed plates; incomplete casting of the battery straps resulting in dropped plates; defects in paste mixing, which lead to poor paste adhesion; and contamination of the paste or electrode. To overcome these inherent problems of lead-acid batteries, a battery management strategy is typically tailored to the type of battery and its application. For example, VRLA batteries in conventional double-conversion UPS systems are float charged, that is they are continuously supplied with a low charging voltage. The circuit topology necessary for this can cause a significant amount of heat to be continuously generated in the batteries. In flooded lead-acid batteries made with lead-calcium grids, the batteries are not continuously float charged. Nevertheless, other battery maintenance strategies may be important. For example, battery temperature may need to be tightly controlled to reduce corrosion that destroys the battery's capacity to generate current. Limiting the depth of discharge is important in controlling electrolyte stratification in flooded lead-acid batteries. Stratification is the increase of electrolyte specific gravity at the bottom of the battery, and techniques to ameliorate stratification can be difficult and/or costly. Thus, current secondary power source system technology suffers from a variety of drawbacks.
Accordingly, it is desirable to have improved devices and techniques for providing secondary power sources for use as backup power to grid-supplied electrical power.